Cartoon Dialog:
Setting: Elmer and Doctor TreeAge, both standing in his doctors office.
Doctor TreeAge: I’m sorry to say that you have CDO.
Elmer: What’s CDO?
Doctor TreeAge: It’s like OCD but with the letters in alphabetical order the way they should be.

In Case Your Friend Didn’t Get The Joke:
Hey, we know you got the joke, but in case your friend didn’t, this is how you can explain it.  OCD is Obsessive-Compulsive Disorder and is an anxiety disorder where people do repetitive things, or are compelled to put objects in a certain order even if it is not necessary.  So when Doctor TreeAge is putting the OCD abbreviation letters in the alphabetical order of CDO, he is exhibiting Obsessive-Compulsive Disorder.  The joke is the Doctor is exhibiting signs of OCD as he is telling the patient that the patient has OCD (chuckle).

OCD can be from mild to severe.  You probably know someone that has it.  For more information, here’s a much better explanation of the disorder on Wikipedia… http://en.wikipedia.org/wiki/OCD.